Papua New Guinea health authorities have confirmed that the country is experiencing its first polio outbreak in 18 years, after one case of the virus was confirmed in an urban settlement in the country's second largest city.

Key points:

The virus was first detected in late April, and confirmed in late May

Experts say the virus is now "circulating" in the affected community

PNG was certified polio-free in 2000, and has not had a case since 1996

PNG's Department of Health is working alongside the World Health Organisation (WHO) to respond to the outbreak of the potentially deadly virus, which can cause lifelong paralysis in children.

There has been one confirmed polio case in the Lufa Mountain settlement in Lae city, which was first detected in late April.

The victim was a six-year-old boy who developed weakness in his lower limbs, later confirmed to be the result of a polio virus infection.

"We are deeply concerned about this polio case in Papua New Guinea, and the fact that the virus is circulating," the country's health secretary Pascoe Kase said in a statement.

"Our immediate priority is to respond and prevent more children from being infected."

Authorities have confirmed that the polio virus is circulating in the Lufa Mountain settlement, after experts isolated the dangerous virus in stool samples from two healthy children there.

Polio spreads through the accidental consumption of faeces of an infected person, usually through contaminated water or food.

The WHO's PNG representative Luo Dapeng told the ABC's Pacific Beat program that settlements like Lufa Mountain face a greater risk of infection.

"This is an area which has poor coverage of the polio vaccine, poor sanitation, very crowded conditions. That is a high risk area," he said.

Most of the people who live in urban settlements in PNG's cities and towns are migrants from rural areas, who move to urban areas seeking work.

The country has not had a case of polio since 1996, and was certified as polio-free in 2000.

Low vaccination rate an issue

The version of the virus circulating in the settlement is a "vaccine-derived poliovirus", meaning it is a mutated version of the weaker polio virus used in vaccinations.

Vaccine-derived polioviruses are rare, and tend to occur in populations with very low vaccination rates. They develop when children who have not been vaccinated come into contact with the excrement of vaccinated children, and are exposed to the weaker virus.

This is not necessarily a bad thing as this exposure can give "passive" immunity to those unvaccinated children — but it can become very dangerous in communities with low vaccination rates.

The weak polio virus ends up infecting more people, stays alive longer, and eventually mutates into a more dangerous form of the virus capable of causing paralysis.

The WHO and PNG authorities have launched an emergency immunisation campaign to try to prevent other children from becoming infected.

Lae is located in PNG's Morobe province, which reportedly had a polio vaccination rate of 61 per cent prior to the outbreak. Authorities said 845 children had been immunised since the virus was first detected.

The province-wide campaign will also target neighbouring Eastern Highlands and Madang provinces, and will include two rounds of immunisation starting from July 17.

The WHO said it has mobilised $680,000 to support the PNG Government's actions.